The use of nuchal translucency measurement and second trimester biochemical markers in screening for Down's Syndrome

Objective To assess the effectiveness of antenatal screening for trisomy 21 by first trimester sonography followed by second trimester biochemical scr eening Design Retrospective five-year review. Setting Maternity unit of a university hospital. Population An unselected group of 7447 pregnant women who had a first trime ster scan and nuchal translucency measurement in our unit after January 199 5 and had an estimated date of delivery before I January 2000. 11.9% were g reater than or equal to 37 years old. A subgroup (n = 4864) also had second trimester biochemical testing by alphafetoprotein and free beta -human cho rionic gonadotrophin. Main outcome measures Prenatal and postnatal diagnosi s of trisomy 21. Results There were 23 fetuses affected with trisomy 21. The overall prenata l detection rate was 87% (20/23, 95% Cl 66% to 97%) and we performed invasi ve procedures in 8.5% of our population. First trimester sonography identif ied 74%, (95% CI 51.6% to 89.8%) of affected fetuses. Second trimester bioc hemical screening detected half of the fetuses with trisomy 21 which were m issed by first trimester screening, increasing the sensitivity to 90.5% (19 /21: 95% Cl 69.6% to 98.8%) for an invasive procedure rate of 4.2% performe d in screened positive women. However., the positive predictive value of th e biochemical test was very low (0.5%). In screen negative women, karyotypi ng for advanced maternal age did not detect any affected fetuses. Conclusion First trimester nuchal translucency measurement is an effective screening test for the prenatal detection of fetuses with Down's Syndrome. Although the measurement of biochemical markers in the second trimester can detect additional affected fetuses this may be outweighed by the delay in diagnosis, the extra visits and cost so that the right time for biochemical screening is most likely to be in the first trimester.